The present invention relates to a driving method for an electro-optical display containing an electrochromic material held in two electrode carrying support plates to manifest reversible variations in the light absorption properties when current is supplied.
More specifically, the present invention relates to a modified driving method of a constant voltage type for an electrochromic display device, which shows minimum power dissipation and rapid response.
Generally, there are three types of electrochromic displays (ECD).
The first type of ECD utilizes an electrically-induced chemical reduction of a colorless liquid to produce a colored, insoluble film on a cathode surface. A typical colorless liquid suited for the first type of ECD is an aqueous solution of the conducting salt, KBr and an electrochromic material, viologen, which produces a purplish film upon electrochemical reduction. See, for example, C. J. Shoot et al, Appl. Phys. Lett. 23 64 (1973).
When a display electrode is held at a potential lower than a predetermined level (a threshold level), the following reaction is conducted to produce a purplish film on the display electrode. ##STR1##
The colored condition is maintained for several hours through several days after the above-mentioned potential is removed as long as the display electrode is electrically separated from the driver circuit (memory effects). Conversely, when the colored display electrode is held at a potential higher than the threshold level, the reaction is conducted inversely so that the colored film is oxidized to be dissolved into a transparent electrochromic material solution.
In the second type of ECD, the color variation is produced by the change in the absorbance of an inorganic solid film formed on electrodes. The inorganic film used in the second type of ECD is the film of the transition metal oxide material such as tungsten oxide (WO.sub.3). Such a film cooperates with an electrolyte. A typical system of the second type ECD is disclosed in B. W. Faughnan et al, RCA Review 36 177 (1975).
The coloration operation in the second type of ECD is caused by injection of protons from the electrolyte and injection of electrons from the electrode. The injection of protons and electrons creates tungsten bronze. The coloration operation is as follows: ##STR2## When the display electrode is held negative, the coloration is conducted. And when the display electrode is held positive, the bleaching is conducted. The memory effect is also expected as the first type of ECD.
An example of the ECD of the second type is described in copending application, ELECTROCHROMIC DISPLAY, Ser. No. 773,774, filed Mar. 2, 1977 by Kozo Yano and Hisashi Uede and assigned to the same assignee as the present application.
The third type of ECD employs the EC material similar to that employed in the second type and a solid state electrolytic film through which ions can travel but electrons can not travel. An example of the third type of ECD was disclosed in U.S. Pat. No. 3,521,941 entitled "ELECTRO-OPTICAL DEVICE HAVING VARIABLE OPTICAL DENSITY" on July 28, 1970.
The above-mentioned ECD has the following characteristic features, in general:
(1) low voltage drive (below several volts) PA0 (2) memory effects are expected, which maintains the colored state for several hours through several days after the applied voltage is removed PA0 (3) low energy consumption (for a single cycle of coloration/bleaching the energy consumption is several through several tens mj/cm.sup.2 PA0 (4) the degree of the coloration is determined by the charge amount flowing therethrough PA0 (5) contrast is very high and is independent of the viewing angle PA0 (6) high visibility is expected even when the ambience is bright, because ECD is a passive display PA0 (7) display surface is flat and display pattern configurations can be arbitrarily selected PA0 (8) driver circuit can be implemented with semiconductor elements PA0 (1) The potential of the counter electrode must be stable even when the number of display electrodes in the coloration state varies. PA0 (2) The over potential due to the reaction at the counter electrode must be held small. PA0 (3) The resistance value of the lead electrode connected to the display (or segment) electrode must be small and inversely proportional to the size of the corresponding display electrode. PA0 [A] Upon every change of the display information, every display electrode is once placed into the bleached condition. Thereafter, the coloration operation is conducted to selected display electrodes. This method consumes large energy. Moreover, a time period of (.tau..sub.W +.tau..sub.E) is required to complete the change of the display information when time periods of .tau..sub.W and .tau..sub.E are required for completing the coloration operation and the bleaching operation, repsectively. PA0 [B] The coloration voltage or the bleaching voltage is applied to only the one or more display electrodes which are not common to the two display patterns in the transition of a visual display from a specific display pattern to another. No voltage is applied to the one or more display electrodes common to the two display patterns. The energy consumption is greatly reduced. A typical driver circuit for conducting this method is described in copending application, DRIVING TECHNIQUE FOR ELECTROCHROMIC DISPLAYS OF THE SEGMENTED TYPE, Ser. No. 751,819, filed Dec. 17, 1976 by Hisashi Uede, Yasuhiko Inami, Hiroshi Hamada, Tadanori Hishida and Hiroshi Nakauchi and assigned to the same assignee as the present application.
Generally, there are three types of driving methods for ECD. That is, the ECD is driven in a method either one of the constant potential type, the constant current type, and the constant voltage type.
In the constant potential type, a reference electrode is provided for maintaining a display electrode potential at a predetermined value. The display quality is very high, but a driver circuit becomes complicated and the power supply voltage is not effectively used.
In the constant current type, current flowing through a unit area is held at a fixed value. The coloration degree can be controlled to a desired value. However, a driver circuit is not suited for mass production, since the constant current value must be set by taking acccout of display sizes of respective segment electrodes employed in the ECD. Moreover, the power supply voltage is not effectively used, because the transistors employed in the driver circuit of the constant current type must operate in the active region.
Examples of the driver circuits of the constant potential type and the constant current type are described in copending application, CONSTANT CURRENT SUPPLY DRIVER FOR ELECTROCHROMIC DISPLAYS OF THE SEGMENTED TYPE, Ser. No. 800,008 filed May 24, 1977 by Yasuhiko Inami, Tadanori Hishida, Kozo Yano, Hiroshi Hamada, and Hiroshi Nakauchi and assigned to the same assignee as the present application.
In the constant voltage type, a constant voltage is applied across a display electrode and a counter electrode for a predetermined time period. The driver circuit can be simplified, and the power supply voltage is effectively used since the transistors employed in the driver circuit of the constant voltage type operate in the saturated region. However, the following characteristics are required to obtain high visibility.
The present inventors have developed an ECD cell which fulfills the above requirements. Therefore, the present invention relates to a modification of the drive method for ECD of the constant voltage type.
Since the ECD has the memory effects and the coloration degree is dependent on the current amount flowing through the display electrode, the application of the coloration voltage must be controlled not to be superimposed to obtain a uniform display. There have been proposed the following two methods for precluding the superimposed application of the coloration voltage.